This invention relates to a method of recovering paper pulp from sugar cane bagasse, to the paper pulp so obtained and to newsprint made therefrom.
Due to steadily rising standards of living in all countries, the production of pulp and paper from sugar cane bagasse is of increasing importance, especially in countries with limited pulp wood resources and considerable sugar cane cultivation, so bagasse is, for practical purposes, the sole source of fibrous raw material.
In view of the growing desire of people for information, the possibility of producing newsprint paper from sugar cane bagasse is of extraordinary importance in those countries.
Since newsprint paper is a high volume product, low production and finishing costs are primary considerations. The development of technology is such that newsprint paper can be produced economically and efficiently only using modern big paper machines which operate at very high speeds.
Newsprint paper is usually produced exclusively from mechanical pulp made from soft wood to which is added certain amounts of chemical pulp. Due to its high yield, mechanical wood pulp can be produced at low unit production cost. The addition of chemical pulp results in suitable strength properties and runability. Usually, unbleached sulphite or bisulfite pulp grades, semi-bleached sulfate pulp, etc., are used. Other pulp types, e.g., chemiground wood or semi-chemical high-yield pulps, are used only in a few paper mills.
Raw materials used for production of newsprint paper have to be of a specified quality. When one component has to be substituted for another pulp grade due to material shortage or high price, the substitute should be of proper quality for processing with modern machines. These properties include runability on the paper machine and on different printing machines as well as quality of the paper and the printing. Thus, a substitute fibrous material must not cause any difficulties in sophisticated modern production methods.
The quality of fibrous materials for newsprint paper production is usually tested by measuring certain physical properties, e.g., dewatering properties, mechanical strength at certain freeness, breaking length, tensile, tearing strength and folding endurance. For newsprint paper, initial wet strength is of decisive importance. This property relates to the behavior of the wet web at the transition from the press section to the dryer section of the paper machine. Other properties taken into consideration include runability on the paper machine, e.g., sticking of the wet paper web to the press rolls. Furthermore, brightness and opacity of the pulp are of great importance.
A conventional pulp mixture suitable as newsprint furnish, consisting of about 80% mechanical pulp from spruce and 20% bleached chemical pulp from spruce or pine wood, has the following test values at a freeness of 30.degree.-40.degree. Schopper-Riegler
______________________________________ breaking length: 3000-5000 m. initial wet strength: 30-60 g. (medium quality) 60-80 g. (good quality) 80-100 g. (superior quality) ______________________________________
According to present technology, the above-given properties are required for a pulp furnish processed at paper machine speeds up to 1000 m./min. at a weight of about 55 g./m..sup.2.
Test values of the finished newsprint paper are slightly different from the values of the pulp raw material. Typical data for newsprint paper are in the following range:
______________________________________ brightness 67 to 60% MgO (Elrepho) density 0.5 to 0.6 g./cm.sup.3. breaking length 2500 to 4500 m. tear 30-60 cmp./cm. bursting strength 1.0-1.5 kp./cm.sup.2. ______________________________________
The final criteria of suitability of newsprint paper are mainly runability in the printing machine and printing quality. Normally, paper is passed through the printing machine at a velocity of about 8-10 m./sec. Tear is of primary importance, since rupture of the paper is caused by lateral defects of the paper web. Pulp produced from sugar cane bagasse has to meet all requirements above to be suitable for newsprint production.
Pulp of sugar cane bagasse has been produced for several years on a commercial basis (see: Atchison "Utilization of Sugar Cane Bagasse in the Pulp and Paper Industry", Paper Trade Journal, 1952). The procedure used is similar to the pulping of wood. A special feature of the raw material preparation is the depithing operation, because the pith in the sugar cane stalk is unsuitable for pulp production.
Pulp produced from the bagasse fiber fraction has been used successfully for the production of different paper grades, e.g., in bleached pulp for the production of medium-fine and fine writing and printing papers.
Although scientists had concluded that the morphological structure of the sugar cane stalk under proper conditions should result in a fiber product suitable for newsprint paper furnish, the production of newsprint paper from 100% bagasse fibers on a commercial basis has been unsuccessful. (See: Pulp and Paper Int., June 1972, page 70).
Numerous efforts and test runs have been carried out by different companies. Tetlow describes a process (Pulp and Paper Int., May 1972, page 58), commonly known as the Cusi Process. Another procedure called Peadco-Villavicencio Process is described in Pulp and Paper Int., January 1972, pages 41 & 42.
Both of these processes were developed by intensive work based on precise knowledge of the production technology for bagasse pulp for fine papers. However, a fibrous material suitable for the production of newsprint paper on 100% bagasse basis could not be obtained.
The Peadco-Villavicencio Process (Tappi CA Report Nr. 40, 1971, pages 137 ff) consists essentially of the following steps:
(a) Bagasse storage by any known method;
(b) two-stage depithing, consisting of moist depithing and wet cleaning;
(c) prehydrolysis with water at 8.5 atm. and 175.degree. C.;
(d) digestion at pH 8 using 1% sodium silicate and 2% sodium bisulfite at 175.degree. C.; and
(e) washing, screening, cleaning and concentrating by any known method.
There is no bleaching step. Sodium silicate added during digestion is alleged to act as oxidizing agent. Yields of 80%, calculated on bone-dry depithed bagasse fiber, are claimed.
The Cusi Process consists of:
(a) Bagasse storage, not specified; drying of bagasse is optional;
(b) depithing;
(c) two-step digestion consisting of an impregnation step and a cooking step with mild pulping conditions;
(d) classification of pulp fibers into two fractions;
(e) severe mechanical treatment of the fiber fraction of lower pulping degree and subsequent re-mixing with the fiber fraction of higher pulping degree; and
(f) intensive bleaching with 10% sodium hypochlorite.
Disadvantages of the two processes above include:
(a) Both known processes employ two-stage digestion, which requires a considerable capital investment for digestion apparatus.
(b) Prehydrolysis by the Peadco Process is carried out under uncontrolled conditions and may lead to varying results depending on random parameters.
(c) The alleged oxidation of lignin by the sodium silicate in the Peadco Process is not in agreement with well established chemical theories.
(d) All plant trials for the production of newsprint by the Peadco Process have failed.
(e) The Cusi Process produces fairly dark pulps which have to be subjected to intensive bleaching. High material losses and problems with the disposal of chlorine-containing bleaching effluents result. Because dissolution of organic material occurs in the bleaching rather than in the cooking stage, recovery of chemicals from the spent cooking liquor is impractical and uneconomical.
(f) A low pulp yield is obtained, which may be 15% lower than by the process of the present nvention, by the Cusi Process.
(g) Mechanical pulp strength is insufficient by either the Cusi or Peadco Process. Breaking lengths obtainable vary between 3000 and 5000 m.; tearing strength is between 30 and 70 cmp./cm.
(h) Reports in the scientific literature (Pulp and Paper International, January, 1972, pages 41 ff) indicate that neither the Cusi nor the Peadco Process permits production of newsprint from 100% bagasse fiber. In both cases considerable amounts, of the order of 15%, of chemical long-fiber pulp had to be added.
Test runs in commercial pulp plants have shown that bagasse pulp provides a substitute for the mechanical wood pulp in the newsprint furnish, but 5-15% of high-grade chemical pulp had to be added. The ultimate aim is production of a bagasse pulp having qualities which allow production of newsprint paper from bagasse pulp only. This is of particular importance to areas having limited wood resources.
In producing pulp suitable as the sole raw material for newsprint paper utmost care should be taken that all favorable properties characteristic of the sugar cane stalk are conserved as completely as possible. Commonly known processing methods damage the bagasse fibers in the initial processing steps, storage and depithing, as well as in the pulping and the bleaching processes.
Due to a residual sugar content of about 2%, the bagasse is exposed to bacterial decomposition during transport and storage under moist conditions, resulting in decreased fiber strength. Bacterial attack can be reduced by addition of black liquor, addition of propionic acid, or by drying raw bagasse prior to storage. However, fiber damage was not completely eliminated, whether caused by bacterial attack or by hornification during drying.
Unsuitable storage conditions can initiate the formation of greyish-green to intensive brown components which resist bleaching under normal cooking and bleaching conditions. In these cases, the pulp can be bleached only by use of a drastic, expensive bleaching process.
Before pulping, pith, which decreases strength properties and increases chemical consumption in the cooking stage, should be removed as completely as possible. Removal of pith is not necessary to this high degree for some other pulp grades. Since pith cells cling firmly to the fiber, mechanical energy has to be applied to separate them and may result in mechanical damage of fibers. Therefore, optimum conditions for depithing have to be developed.
Owing to heterogeneity and varying density over the cane stalk profile, bagasse fiber bundles are attached more or less by the mechanical depithing procedure and the chemical pulping reaction. The varying density of the fiber system of sugar cane is the basis for producing pulps having a spectrum of degrees of pulping, similar to conventional newsprint furnish consisting of mechanical and chemical wood pulp.
Thus, a pulping procedure mild enough to retain the specific characteristics of the different fractions should be used. Highly alkaline solutions may cause stabilization of deeply colored complexes which cannot be bleached by a mild one-step bleaching process.
Thus, there is a continuing need for a process in which the initial brightness of the bagasse fibers during raw material preparation, storage and cooking is preserved in the unbleached pulp in order to obtain a bright ready stock by a mild bleaching procedure.
The objects of the present invention are:
(a) to provide improved bagasse paper pulp, especially of such properties that it may be used as the sole fiber component for the production of newsprint.
(b) to provide a process by which a bagasse pulp suitable as sole fiber component for the production of newsprint paper may be manufactured;
(c) to provide a process in which bagasse fibers are not subjected to deterioration by irreversible hornification caused by drying;
(d) to provide a process in which attack of the bagasse by bacteria during storage is avoided, i.e., in which bacterial degradation and, hence, loss of strength are avoided;
(e) to provide a process in which brightness losses and coloration of bagasse fiber, and formation of more or less insoluble colored complexes are avoided during storage and pulping operations;
(f) to provide a process including pulping conditions of high specific selectivity for bagasse fiber while retaining or improving the original brightness of the fiber component;
(g) to provide a process in which the inherent strength properties of bagasse fiber after pulping are retained during bleaching;
(h) to provide a process with reduced loss of fibrous material during processing of bagasse fibers;
(i) to provide a process which causes a minimum loss of inherent opacity of bagasse fiber;
(j) to provide a process in which the yield of bagasse is increased and the strength value is increased; and
(k) to provide a process which reduces the overall quanity of chemicals required, e.g., in the bleaching step, since the usual green/brown coloration appearing during storage is reduced, whereby a brighter unbleached pulp is obtained.
Upon further study of the specification and appended claims, further objects and advantages of this invention will become apparent to those skilled in the art.